This invention relates to coated sheet material, and particularly to the production of microcapsule coated paper for use in pressure sensitive copying systems.
One such copying system, known as a transfer system, comprises an upper sheet, known as a CB sheet, coated on its lower surface with microcapsules containing a solution of a colourless colour former and a lower sheet, known as a CF sheet, coated on its upper surface with an acidic colour-reactive material, for example a clay, a phenolic resin or certain organic salts. For most applications, a number of intermediate sheets, known as CFB sheets, are also provided, each of which is coated on its lower surface with microcapsules and on its upper surface with acidic colour-reactive material. Pressure exerted on the sheets by writing or typing ruptures the microcapsules, thereby releasing the colour former solution onto the acidic material on the next lower sheet, and giving rise to a chemical reaction which develops the colour of the colour former.
Another such copying system, known as a self-contained system, comprises at least one sheet coated on its upper surface with both microcapsules and acidic colour-reactive material. Again, pressure exerted on the sheets by writing or typing ruptures the capsules, thereby releasing the colour former solution onto adjacent colour reactive material and so developing the colour of the colour former.
As well as microcapsules, a coating composition for producing CB, CFB and self-contained sheets generally contains a binder and a so-called stilt material. The latter is a protective agent for preventing premature rupture of the capsules, for example during storage and handling of the sheets. The stilt material is particulate in nature. The particles thus support the sheet if it is subjected to relatively light pressures, for example those involved in handling and storage, and thus prevent the microcapsules from being subjected to pressure which might rupture them. However, if high pressures are applied, as is the case during writing or typing, the protection afforded by the stilt material is insufficient to prevent microcapsule rupture.
The binder serves to enhance the adhesion of the microcapsules and the stilt material particles to the base paper.
In the copying systems described above, it is desirable that the microcapsule-containing coating extends completely and evenly over the whole area of the CB, CFB and self-contained sheets, so as to ensure that all areas of the sheet are capable of generating a copy. It is also desirable for economic reasons to use a low coatweight of microcapsule-containing composition. The achievement of the desired combination of complete even coverage and low coatweight presents practical and economic problems, as will now be discussed.
Hitherto, it has been a common practice to apply microcapsule-containing coatings to the paper as aqueous dispersions at a low solids content, and then to meter the coating applied using an air-knife. A low solids content coating composition, e.g. 18% solids content, permits the use of a fairly thick wet film, which ensures complete coverage and results in the desired low coatweight once the water has been removed. Moreover, an acceptably even coating is obtained, since air knife metering results in a coating which tends to "follow" the "hills" and "valleys" in the paper, rather than filling in the "valleys" and leaving the "hills" with a low coatweight. However, the procedure just described has a number of disadvantages. Firstly, the use of a lower solids content coating composition means that large amounts of water have to be removed from the coated web of paper (a high solids coating composition cannot be satisfactorily air-knife coated, since its higher viscosity means that the flow properties of the mix are not adequate to permit metering by a jet of air). Secondly, the speed at which coating may be carried out is limited by the problem of "misting". This is due to the fact that the metering jet of air tends to blow droplets of liquid off the web giving rise to a "mist", which may subsequently be redeposited either on the paper, which gives an uneven coating pattern, or on the tip of the air knife, which disturbs the evenness of the metering jet of air and thus gives rise to an uneven coating pattern. The problem becomes progressively worse at higher coating speeds, since the pressure of the metering jet of air is greater. Skilful operation can minimize this problem, but it does represent a constraint on high speed operation. Thirdly, particles of stilt material tend to be selectively blown off the wet coating on the web, although again this problem can be overcome to some extent by skilled operation of the coater. Fourthly, air knife coating is only effective for use on webs of up to a certain deckle, so far as we are aware.
If it is sought to overcome the above described problems by using blade metering rather than air knife metering, it is difficult to achieve the desired combination of low coat-weight and complete even coverage, with a good coating pattern. In order to achieve complete coverage, it is still necessary to apply a fairly thick wet coating, but a blade is not normally usable to meter a coating having a low viscosity, for example below about 200 cP Brookfield. A low solids content microcapsule mix of, for example 18% solids has a lower viscosity than this. If a higher solids coating composition is employed, in order to achieve a higher viscosity, either an uneven coating pattern results or an uneconomically high coatweight has to be applied to achieve an even pattern after water removal. This problem is exacerbated by the fact that although blade metering gives a smooth surface, it may result in an uneven film thickness, since in contrast to an air knife the "valleys" in the paper are filled in and the "hills" have a low coatweight.
It might be thought that the viscosity could be simply increased by the addition of a thickener. However, a thickener does not increase the wet film thickness and hence an uneconomically thick wet coating would still be needed in order to ensure complete coverage.
The presence of particulate stilt materials in the coating mix leads to further problems in the use of a blade coater. Firstly, the blade may cause selective removal of stilt material, which is then returned to the coating pan with the excess coating removed by the blade. In consequence, the viscosity of the coating composition in the coating pan increases, and control of lightweight coatings becomes increasingly difficult. This problem is particularly serious when cellulose fibre floc is used as the stilt material (cellulose fibre floc has hitherto been probably the most widely used stilt material, although other materials have been proposed, for example, wheat starch, granular synthetic polymers and certain mineral materials). Secondly, particles of stilt material tend to pack beneath the tip of the blade which leads to low coatweight streaks in the coating. Moreover, parts of the blade may lift intermittently to release the packed particles of stilt material, which leads to the production of "strips" of excessively high coatweight.